The present invention relates to the full-scope real-time simulation of the dynamic operation of a nuclear powered electrical generating plant for training plant operators.
The increasing demand for well trained power plant operators together with the complexity of modern day power plants, has led to the realization that the simulator is the most effective tool for such training.
Also, with advancements in nuclear power plant technology, experienced operators from time-to-time need retraining in order to be competent. An actual nuclear plant cannot provide the operator with the required experience, such as starting up, changing load, and shutting down, for example, except after years of experience; and even then it is unlikely that he would observe the effect of important malfunctions and be able to take the best corrective procedures.
Although simulators have been used for many years, in power plant design, it is only recently that they have been used for power plant operator training. An article in the July 22, 1968 issue of Electrical World, entitled "Nuclear Training Center Using Digital Simulation" briefly describes the installation of a boiling water reactor plant simulator. An article in the same publication in the Oct. 6, 1969 issue entitled "Huge Simulator to Ready More Reactor Operators" discusses the proposed installation of a pressurized water reactor simulator. In Volume 10, No. 5 of the publication "Nuclear Safety" published during September and October 1969 is an article entitled "Training Nuclear Power Plant Operators With Computerized Simulators", and in the June 1972 issue of the publication "Power Engineering" there is an article entitled "Simulators" which describes a number of power plant operator training simulators presently in use or proposed.
Design simulators usually cover only a small part of the process, and may run slower or faster than real-time; while training simulators must operate and respond in a manner identical to the actual plant. A design simulator may involve only a narrow range of conditions, while a training simulator must simulate from "cold" shutdown to well beyond normal operating conditions. A design simulator usually involves only the major process, while a training simulator should cover every auxiliary system with which the plant is concerned.
Training simulators presently in use for operator training, which are more or less complete in their simulation utilize a digital computer that is connected to control consoles that are identical in operation and appearance to the plant being simulated. Also, an instructor's console is connected to control the simulator, introduce malfunctions, initialize the simulated plant at selected states of operation, and perform other functions useful for training purposes and control of the simulator.
Also, the simulation of the power plant must be of sufficient detail and accuracy that the operator cannot distingish between the behavior of the simulator and that of the actual plant under all normal and abnormal operating situations.
In the auxiliary and engineered safeguards systems, of the plant, for example, there are several complex non-linear fluid networks which must be simulated to operate in real-time under various operating situations. It is desirable to simulate such systems to eliminate iterative techniques. One way of accurately simulating such systems accurately with relative simplicity is to perform the calculations in accordance with an analogous electrical network, with the non-linearities applied by coefficient variation during successive time steps. Thus, linear equations can be solved with an initialized constant to obtain a new dependent variable, such as flow, from the linearized solution. Then, on a subsequent time step the new dependent variable is used to calculate a new set of coefficients where the non-linearity is included in the new set of coefficients for calculating a new updated variable. Thus, during each time step a value is calculated that is closer to the value that would be obtained by a complicated non-linear solution. To eliminate occasional numerical instability, an added correction term which is directly analogous to inertia in a fluid system can be used.
Further, with respect to boration of the reactor coolant loop, only the reactor senses the boron concentration. However, the time delays for the reactor to be effected by the injection of boron into different parts of the reactor coolant system loops are appreciable. Thus, in order to have accurate training simulation, this time delay should be simulated. For this purpose, it is desirable to simulate the reactor, reactor coolant loops and primary side of the steam generator as one tank. Should one of the reactor coolant loops be isolated, the equivalent volume for that loop can be removed from the simulated tank with respect to boron concentration. The isolated can be considered as a separate tank with the only connection between the two tanks being a mini-bypass flow around a cold leg stop valve of the loop, and the other leg stop valve if open. This path can then be used to balance the boron concentration in the isolated loop prior to cutting it into the system.
The actual safety injection system of a nuclear power plant consists of many parallel flow networks sharing many common components. These parallel paths have pressure heads defined at each end; and with the exception of severe accident conditions when several flow paths are called automatically, the specific flow path in operation is in accordance with a particular operating condition. In simulating the safety injection system, it is desirable to model the system into several different routines. Thus, upon the opening of the simulated valves, only the paths connected with those valves are called into operation. This saves computer time and unnecessary calculations. For each flow network, there are boundary conditions at both ends; and one desirable manner of solving for flow is to make the calculations in accordance with the electrical network analogy together with the introduction of inductance to inetgrate the flow from zero to steady state. This can be accomplished by using the pressure head at one end, to calculate flows and pressures down the loop and the expected pressure at the opposite end. Should the expected pressure differ from the pressure head at the one end, the difference can be divided by the inductance of the loop and converted to the required incremental flow and added to the flow distribution. Then the pressures can be recalculated until the pressures at each end of the particular path of the safety injection system are equal.
The residual heat removal system, like the safety injection systems has pressures defined at several boundaries. Two pressures are defined at the inlet and outlet of the reactor coolant loops, respectively; a third on the letdown line to a volume control tank and a fourth at a storage tank. Additionally, the residual heat removal system loop fans out to three additional loops, two of which are for cooling the coolant in the reactor coolant loop using residual heat removal pumps and heat exchangers, and a third, which is used primarily during start-up to control pressurizer pressure through the letdown line to the volume control tank. There is also a feedback path around each of the residual heat removal pumps and heat exchangers which are used primarily for warming the pumps prior to their use for cooldown. This system can be simulated in the same manner as the safety injection system. Additionally, it is necessary to simulate the thermal mixing of the three cooling paths in order to obtain accurate input and output enthalpies to the reactor coolant loops and letdown line.
The chemical and volume control system also can be subdivided into several sections for the purposes of simulation. However, in this regard, it is necessary for accurate simulation to consider the instability of the pressure regulated valve at low letdown flows. One advantageous way of effecting such a simulation is to simulate a small valve in parallel to provide a minimum valve coefficient for the controller. In actual plants, this valve is one that never closes completely. Further, in the simulation of the heat exchangers, it is possible for the flows to go to zero depending on the inlet and outlet valve positions, which causes thermal runaway on the low flow side. One way of preventing such an occurrence is to provide that the exit temperature of the low flow side does not exceed the inlet temperature of the other side of concentric flow heat exchangers or the outlet temperature for parallel flow heat exchangers.
The demineralization in the chemical and volume control system for the sake of simplicity can be simulated as pressure drops; and the automatic regulator for the volume control tank can be simulated such that nitrogen enters the volume control tank under low pressure and is relieved if it exceeds a maximum amount, in order to maintain a constant pressure on the tank.
The seal flow system which is used to prevent hot radiated water from entering the reactor coolant pump bearings and seals, should be simulated because of its importance to the system. This system is also used to pressurize an isolated reactor coolant loop by charging around the thermal barrier. However, it is desirable in such simulation to obtain correct calculated outputs for the large number of indicating devices on the control panel, and to keep the simulation as simple as possible. In keeping with this objective, it is desirable to simulate explicitly fast flows and pressure transients because of the small differential pressures, and to simulate the thermal effects on pump bearings and thermal barriers as lags.
To be complete, the component cooling system, which distributes cooling water to the various cooling systems, should be simulated. The system can be modeled as several continuous flow loops. The flow can be made dependent on the number of component cooling pumps in operation and their speed. The amount of surge flow can be made dependent on the change of density of the component cooling water due to thermal heat transfer at heat exchangers or cooling areas of the component cooling system loop.
To complete the simulation of the primary auxiliary systems, the accumulator system, which injects water into the reactor coolant loop automatically upon a decrease in pressure should be modeled in a simplified manner.